A downhole zonal isolation tool often is used for stimulation or service work upon a well. For example, the zonal isolation tool is a bridge plug, frac plug, or packer for bridging a hole or gap of a metal tubular such as a well casing.
The zonal isolation tool typically has an internal elongated mandrel and a circular array of slips mounted on the mandrel at each end of the tool. Each slip has an outer surface adapted for engagement with the internal surface of the well casing. Each slip also has an inclined inner surface. Each array of slips is disposed next to a respective conical ring mounted on the mandrel for sliding under the inclined inner surfaces of the slips in the array. In the middle of the zonal isolation tool, rings of elastomeric sealing material are mounted on the mandrel between the conical rings.
When a setting tool pulls the mandrel in the longitudinal direction, the rings of sealing material expand outward in the radial direction to seal the well casing. In addition, the conical rings slide under the slips and force the slips outward in the radial direction into engagement with the well casing. The slips lock the zonal isolation tool in place inside the well casing in such a way that the rings of sealing material remain in compression for sealing the well casing when the setting tool is removed.
The zonal isolation tool can be designed to be retrievable and reusable after it has been set in the well casing. However, the zonal isolation tool is most economical to manufacture when it has been constructed to become permanently set in the well casing so that it must be drilled out destructively to unseal the well casing.
Traditionally, such a drillable zonal isolation tool has been made of a cast iron mandrel and cast iron slips.
A number of downhole tool makers have replaced the cast iron components of the zonal isolation tools with composite components of epoxy fiberglass. The composite components can be drilled out faster than cast iron, and the drilled-out chips of composite material are lighter than cast iron chips so that the composite chips are more easily flushed out of the tubular member with drilling fluid. The composite downhole tools are also lighter than the cast iron downhole tools and can be used in both high and low pH environments. Details of construction of such composite zonal isolation tools are found, for example, in Turley et al. U.S. Pat. No. 6,712,153, issued Mar. 30, 2004, incorporated herein by reference, and in Sutton et al., U.S. Pat. No. 6,976,534 issued Dec. 20, 2005, incorporated herein by reference.
When set by a setting tool, the composite mandrel must sustain tension in the longitudinal direction of up to about 12,000 psi, as well as compression in the radial direction of up to about 40,000 psi. The composite mandrel must also sustain internal pressure of well bore fluid. Due to these forces, the fiber reinforcement of the composite material should have a degree of directional orientation.
The composite mandrel has been fabricated from a composite head plug and a pair of coaxial and filament-wound composite tubes. The filament-wound tubes included alternate layers of diagonal and radial fiber, for example, diagonal layers of fiber wound criss-cross at 22 degrees interleaved with layers of fiber wound in a circumferential wrap. Each composite tube was wound on a respective steel mandrel.
The outer cylindrical surface of the inner composite tube was ground to match the inner diameter of the outer composite tube, so that the inner composite tube could be closely fitted into the outer composite tube. The composite head plug was also inserted into the outer composite tube, and the composite head plug and the composite tubes were pinned and glued together. Such a composite mandrel was rather expensive due to the cost of the head plug and the cost of the two composite tubes, and the cost of grinding the inner composite tube.